While titanium alloys have been used extensively in aerospace and other applications, the need for relatively lightweight alloys for use at elevated temperatures has increased. For example, the higher performance and higher fuel efficiency of airplanes and aero-engines are leading to the development of aero-engines and airframes operating at increased temperatures and decreased weight. As a result, titanium alloys are being considered for use in the hotter section of engine nacelles or in airframe parts which undergo higher operating temperatures, such as aft pylon components. These developments have led to a need to replace heavy nickel base alloys (and others) with titanium alloys having excellent oxidation resistance and high strength at elevated temperatures, such as, for instance, 650° C., 700° C. or 750° C. or higher.
While titanium alloys such as Ti-6Al-2Sn-4Zr-2Mo-0.1Si and Ti-15Mo-3Al-3Nb-0.2Si have been used to form the airframe or aero-engine components for which oxidation resistance, heat resistance and lightness are required, the oxidation resistant temperature of these alloys is usually limited below 650° C. Thermal exposure at 700-750° C. for prolonged periods leads to severe flaking of components formed of these two alloys. Moreover, the latter alloy has significantly lower strength when service temperatures reach 700-750° C., as it is a near-beta titanium alloy.
Several titanium alloys are noted below which provide varying desirable characteristics, but which are not suitable for the above-noted purpose. The commercial titanium alloys Ti-6Al-2Sn-4Zr-2Mo-0.1Si and Ti-15Mo-3Nb-3Al-0.3Si disclosed in U.S. Pat. No. 4,980,127 are near-beta titanium alloys with very high content of molybdenum. U.S. Pat. No. 4,738,822 discloses a niobium-free near-alpha titanium alloy, Ti-6Al-2.7Sn-4Zr-0.4Mo-0.4Si, which has good strength and creep resistance at fairly elevated temperatures. U.S. Pat. No. 4,906,436 and U.S. Pat. No. 5,431,874 disclose high temperature titanium alloys containing hafnium and tantalum.
U.S. Pat. No. 4,087,292 and U.S. Pat. No. 4,770,726 respectively disclose two niobium-containing titanium alloys, Ti-5.5Al-3.5Sn-3Zr-1Nb-0.25Mo-0.3Si (known as IMI 829) and Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C (known as IMI 834), which show good creep resistance at elevated temperatures. U.S. Pat. No. 6,284,071 discloses a high temperature titanium alloy which normally contains 3.5% zirconium and optionally up to 2.0% niobium. The titanium alloys of the three previous patents contain respectively no more than 1.25, 1.5 and 2.0% niobium and respectively at least 2.0, 3.25 and 2.5% zirconium.
It will be appreciated that producing titanium alloys with excellent oxidation resistance at such high service temperatures (especially at about 700, 750° C. or higher) is extremely difficult. Thus, for example, it is a major leap forward to advance from a titanium alloy capable of operating at 650° C. to a titanium alloy capable of operating at 750° C. with good oxidation resistance and high strength.
The present titanium alloys are useful for this and other purposes, and may provide various desirable physical characteristics other than those discussed above.